The major late transcription unit of the DNA tumor virus, adenovirus, encodes five families of mRNA's, designated L1 through L5, which are transcribed from a single promoter. The members of each family share a 3' end, or poly(A) site, but differ in their splicing patterns. The expression of these mRNA's is primarily regulated post-transcriptionally. Early in infection, transcription terminates between L3 and L4, precluding expression of L4 and L5 mRNA's, whereas late in infection all five families are expressed. In addition, the relative abundance of the different families is regulated by a differential choice of poly (A) sites early versus late. Early in infection, the L1 site is chosen three times as frequently as L2 or L3; late in infection, this ratio is reversed. The goals of this project are to determine the mechanism by which the virus interacts with the cell to regulate expression of these genes. The RNA sequence and protein factor requirements for regulation of poly(A) site choice will be studied by analyzing poly(A) site mutants for function both in vivo and in vitro. The requirements for transcription termination early in infection will be studied by cloning the terminator region into a recombinant vector and assaying its function. In addition, mutagenesis will be used to dissect the sequence and factor requirements for this process. These studies will lead to a better understanding of the growth of the virus in the host cell. Since the virus relies upon the cell's macromolecular synthetic machinery for expression of its genes, the analysis of adenovirus gene expression serves as a model system for those processes in the host.